This invention relates to torsion free or rotation resistant wire ropes made of spiral or helical strands such as used with hoists, derricks, cranes, and similar hoisting equipment. More specifically, the present invention relates to multi-layer ropes in which the layers have opposite directions of lay.
Ropes can be generalized into the groups of non-rotational and rotation resistant. Non-rotational resistant ropes tend to unravel or spin under load if both ends are not fixed. Therefore, those applications wherein one end is not fixed require a rotation resistant rope. Such applications might be where a tower crane lifts concrete buckets or building components from the ground to the top of multi-story buildings under construction. Another example is in oceanography wherein instruments are lowered from a ship to great depths into the ocean. In these situations, the longer the length of rope suspended, the more revolutions the support object on the unfixed rope end will turn if a non-rotational resistant rope is used. In still another example, in multiple reeving situations, a rotation resistant rope is necessary to prevent block spinning. For safety and/or technical reasons, rotation is undesirable.
Multi-layer ropes are known in the art which are substantially torsion free or rotation resistant. In practice, each strand is laid separately in such a manner that the inner layer or layers between the core and the outer or top layer are laid in a direction opposite to the outer layer. This gives the desired relative freedom from twist or rotation of the rope, such as when used to lift an unguided load.
Exemplary of the many patents dealing with rotation rope are U.S. Pat. No. 2,779,149, in the name of Heri Schuller, et al., and U.S. Pat. No. 3,729,921, in the name of Bernard Stroh.
Rotation resistant ropes are generally constructed using more outside strands and heavy cross-laid cores. Because of a generally larger contact angle between cross-laid outer strands and the core, these ropes have lower strength. On the other hand, non-rotational resistant ropes generally have fewer outside strands. These ropes have good crushing resistance and high breaking strengths. Accordingly, the prior art has long sought the development of a rotation resistant rope which will provide good crushing resistance and increased breaking strength comparable to that of non-rotational resistant ropes, while at the same time reducing the rope torque.